I think Magneto was available for a long while from the Steinberg archive, however it's
recently been dusted off and bundled as Magneto 2 as part of Cubase 7.5 so the original
seems to have been removed from the archive.

Do you still have the machine you used to use these on? If so, just do a search for DLL
files called "magneto" and you should find it. But tbh, there are much better things out
there than Magneto now, including some freebies (check out VarietyOfSound's
FerricTDS for example, and Jeroen
Breebaart's Ferox).